Currently, substrates fabricated using techniques combining bonding by molecular bonding (known as “wafer bonding”) and transferring a useful layer onto a support have a zone known as a “peripheral ring”. This peripheral ring is a zone located at the periphery of the support in which transfer of the useful layer has not occurred. The peripheral ring also includes a zone in which the useful layer has been transferred only partially or has disappeared during subsequent treatment due to its poor bonding to the support.
Accompanying FIGS. 1 and 2 are respective cross-sectional and plan views of a substrate that is known to the skilled person under the acronym “SOI,” meaning “silicon on insulator”. FIG. 1 shows a support 1 of silicon onto which a composite layer 2, comprising a layer of silicon oxide 21 surmounted by a layer of silicon 22, has been transferred by molecular bonding.
Peripheral ring 3 defines a substantially annular zone of support 1 onto which the composite layer 2 has either not been transferred or has been transferred poorly or incompletely to a substantial level during transfer of the layer. In the plan view of FIG. 2, it can be seen that this ring 3 is typically of varying width and/or shape, i.e. that the vertical side 200 of the composite layer 2 can be irregular and jagged Islands 2′ of composite layer may exist, which have been transferred onto the support 1, but which are spaced apart from the remainder of the central portion of said composite layer 2. It should be noted that FIGS. 1 and 2 are diagrams that are not to scale both as regards the thickness of the different layers and of the support and as regards the width of the ring.
This ring phenomenon occurs with other substrates, for example those known under the acronym “SICOI” meaning “silicon carbide on insulator” or under the acronym “SOQ” meaning “silicon on quartz”. Other multi-layer substrates, such as those comprising gallium arsenide on silicon (AsGa/Si), also exhibit said ring.
Independently of the diameter of support 1, which typically varies, for example, from 2 inches (50 millimeters (mm)) for silicon carbide to 12 inches (300 mm) for certain silicon substrates, the ring 3 generally varies from about 1 mm to about 4 mm in width, typically plus-or-minus 0.5 mm. Further, the width of the ring 3 can fluctuate, i.e., it can be smaller on one side of the substrate than on the other.
The appearance of the rings during layer transfer has a variety of origins and causes, as discussed below. Certain factors that can cause ring 3 to appear include chamfers on the substrates used, variations in bonding energy between layer 2 and the supports, and finally certain aggressive steps in substrate fabrication methods.
In order to explain how ring 3 typically appears, reference is made to the accompanying FIG. 3 which is a diagrammatic cross-section of a portion of the sides of two substrates bonded together by molecular bonding. A source substrate 4 is preferably a layer from which the future useful layer is to be cut, and a support substrate 5 is selected to receive the useful layer. This figure illustrates the prior art. In the remainder of the description and drawings, the substrates are assumed to be circular in shape, as this is the shape used most frequently. Other shapes can alternatively be used.
The source substrate 4 has two opposite faces that are parallel in most desired applications. In FIG. 3 reference numeral 400 designates what will be referred to herein as a front face of the opposite faces. The front face 400 is intended and prepared for bonding onto the support 5.
The source substrate 4 has a side 41, which can be a peripheral side. Further, the substrate 4 has preferably undergone treatment that forms a zone or region of weakness 42 that defines two portions: a rear portion 46 and a useful layer 43. The useful layer 43 is intended for transfer to the support 5. Throughout the remainder of the description, the expression “useful layer” refers to the transferred layer. The thickness of the useful layer typically depends on whether it is obtained, for example, by a method of implanting atomic species or by abrasive polishing and/or chemical etching as is described below.
In one embodiment, the substrates used both as the source substrate and as the support substrate are commercially available substrates satisfying standardized requirements (for example SEMI M1-0302 standards for a silicon substrate). Those standards are mainly concerned with ensuring that the substrates can be accepted by the equipment of as wide a range of users as possible. According to these standards, at the intersection between the side 41 and the front face 400, the substrate 4 has an annular primary chamfer 44 or primary drop oriented at an angle α, which can be large and close to 45°, with the extension thereof, and more precisely with the central zone 40, which is preferably flat to a high degree of precision, as will be explained below. The primary chamfer 44 extends over a width L in the radial direction parallel to the front face 400. Width L generally varies from about 100 micrometers (μm) to about 500 μm, depending on the substrates used. The primary chamfer 44 is intended to limit the risk of mechanical breakage and notching of the source substrate 4.
In a similar manner to that just described for the source substrate 4, the support substrate 5 also has a front face 500, a side 51 and a primary chamfer 54 according to typically present standards. When substrates 4 and 5 of the prior art are bonded to each other, bonding does not occur at chamfers 44 and 54 because of the magnitude of angle α. The width of the rings can thus be expected to generally correspond to the width L of the primary chamfers 44 and 54. In practice, however, the width of FIG. 3 is typically even wider.
It has been observed that the front face 400 of substrate 4 actually has two zones, namely a first flat zone 40 located substantially at the center of the substrate 4 and hereinafter also termed the “flat central zone,” and a second zone 45 surrounding the first zone 40.
The second zone 45 is a secondary chamfer, which is generally annular, or a secondary drop, forming an angle β with the plane of the flat central zone 40. The secondary chamfer of second zone 45 extends between the flat zone 40 and the primary chamfer 44.
Throughout the remainder of the description and claims, the expression “flat” means a flatness that is suitable for bonding. The expression “central zone” designates a zone located substantially centrally or even at the center of the front face of the substrate and which can be located at various degrees of eccentrically, and most preferably, located slightly eccentrically on the front face 400.
It should be noted that as FIG. 3, the following figures are only diagrammatic in nature and the magnitude of angle β has been considerably exaggerated in the figures for clarification purposes. More precisely, the secondary chamfer 45 a drop that is less sharp than the primary chamfer 44 and that appears during the various substrate shaping steps such as lapping, polishing, and chemical etching, which steps produce an etching and material-removal effect that is greater near the substrate side 41. The secondary chamfer 45 is presently not subject to industry standards. Its width L′ taken in a radial direction varies from about 500 μm to 3000 μm on substrates that are commercially available on the market. Further, the value of angle β also fluctuates, and the secondary chamfer 45 is not flat in cross-section as shown diagrammatically in FIG. 3 but can be domed or irregular in places.
As a result, in practice, and in contrast to the diagrammatic representation of the figures, the side of the source substrate 4 is not formed by a plurality of beveled slopes, but instead by an overall convex shape, typically without edges between the secondary chamfer 45 and the primary chamfer 44 or between the primary chamfer and the side 41. The purpose of the convex shape is to avoid any nicking of the substrate 4.
In a manner similar to that just described for the source substrate 4, the support substrate 5 has a flat central zone 50 and a substantially annular secondary chamfer 55, but has similar irregularities to the secondary chamfer 45.
Molecular bonding is a technique that does not tolerate substantial non-planar surfaces, the existence of secondary chamfers 45, 55 results in poor bonding and layer transfer in the zone of these surfaces, resulting in the appearance of a peripheral ring 3. In addition, a second reason for the appearance of the ring 3 is that the bonding energy between two facing substrate faces fluctuates as a function of parameters such as roughness, flatness and the chemical nature of the surfaces in contact, the presence of particles and impurities, etc. These parameters can also vary in a less controlled manner at the sides of the substrates, thereby also contributing to the formation of the ring 3.
Finally, another possible cause for formation of the ring 3 is the use of certain aggressive or vigorous steps during the substrate fabrication methods.
Methods of fabricating substrates known under the acronym BESOI (bond and etchback silicon on insulator) bond a source substrate onto a support substrate, at least one of the faces of the source substrate being coated with a layer of oxide. The exposed surface of the source substrate then undergoes an abrasive polishing and/or chemical attack etching treatment, followed by polishing until the source substrate becomes a useful layer. In this type of method involving chemical attack (with the risk of partial delamination of the bonding interface), oxidation affecting the lateral and frontal portions of the source substrate, and mechanical abrasive polishing forces, both tend to enlarge the ring 3.
Similarly, in methods involving detachment of a layer by fracture along a zone of weakness, it has been observed that around the peripheral sides collective structure of the bonded substrates, detachment often tends to occur at the bonding interface instead of at the zone of weakness, resulting in the formation of an annular ring 3, sometimes with a large surface area.
Referring again to FIG. 3, in the case in which the zone of weakness 42 is formed by implantation of atomic species, such as by hydrogen implantation, it has been observed that, during subsequent treatment to detach the useful layer 43 from the remainder of the source substrate 4, expansion of hydrogen bubbles exerts a substantially perpendicular force on the surface of the secondary chamfer 45. In the zone of the secondary chamber 45, this force is often not compensated by sufficiently strong bonding since the secondary chamfer 55 of the support 5 is spaced from the secondary chamfer 45 of the useful layer 43 by an angle 213, or the addition of the angles β if the angle of each opposing chamfer is different, and the bonding is thus ruptured. Thus, bubbles are formed at the surface of the secondary chamfer 45 instead of at the surface or edge of the layer 43 being transferred onto the support substrate 5. In other words, bonding occurs but its quality is poor in this area.
A number of disadvantages are associated with the existence of said ring 3. First, it is not possible to fabricate electronic components on the surface of this ring 3. Unfortunately, from an economic point of view, each extra square millimeter of area can make it possible to fabricate a larger number of components per substrate. Furthermore, this ring is irregular as explained above, and its width can vary from one side to the other of the substrate, thus giving rise to problems of reproducibility in the various steps of an industrial process when such a substrate is used in a production facility.
The prior art includes methods of polishing the side of a substrate so as to make it possible to eliminate the ring 3, see for example document U.S. Pat. No. 6,221,774. A method of mechanically polishing sides is also known as used by the supplier SEZ. This method is used on solid silicon substrates after deposition operations, which are known to run the risk of being associated with the effects of material being removed from the sides (known as “lift-off” or “peel-off”), i.e. leading to a high level of particulate contamination. Nevertheless, those methods tend to increase the size of the zone that has no transferred layer at the periphery, thereby restricting the useful area. In addition, finishing operations on the ring can lead to defects at the periphery of the transferred layer.